This paper presents a statistical mechanics-based approach to investigate critical phenomena and size scaling in communication networks. The qualitative nature of phase transitions in the underlying network systems is characterized; and its static and dynamic critical behaviors are identified. Effects of network size, different routing strategies have been analyzed. In all these analyses, phase transition is considered using a single intensive parameter of the communication network system, namely the external packet load. These problems have been investigated by extensive simulation on the model of a wired communication network.